1. I think the best approach is to aim to make the wort you boil them same as your anticipated OG. Then, at the end add the rest of the extract so that when you top up you are back at your anticipated OG.
2. If you re-hydrate the 1 pack, it should be OK. But really, you would like to use 15g.
3. Technically you want to partial mash the Munich. No big deal. Bring all the grains with about 2qts of water per pound to around 152. Turn off the heat and let it steep/mash for about 45 minutes to an hour. You can do a search for partial mash brewing for more details. Heat the rest of the water to about 170 or so and rinse the grains after you are done, this is like sparging.
EDIT: DeathBrewer has a great little thread that demonstrates partial mash brewing:
http://www.homebrewtalk.com/f39/easy-partial-mash-brewing-pics-75231/